Engineering Post Frame Buildings

[medeek]

I'm getting ready to do an analysis of a post frame building and I am still hunting down every resource I can find to educate myself since this is the first post frame building I have ever done. I've managed to acquire a copy of the 1999 Post-Frame Building Design Manual published by the NFBA, which appears to be the defacto standard for pole building engineering. However, online I have found other misc. papers describing a "simplified" method for designing post frame buildings.

I am wondering what others typically use as their reference and what are your thoughts on the simplified methods (Don Bender and Drew P. Mill).

I've also just noticed that the second edition of the NFBA manual has now come available.

http://www.nfba.org/index.html

Has anyone had a chance to purchase it and compare it with the 1999 edition (First Edition)?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

As Mdeek stated, these jobs are typically just very big headaches. Too much detailing for not enough money. I have stopped doing them. What about the whole,"PT wood will rot when embedded in concrete" theory?

 

[msquared48]

That's why I put 6" of gravel in the bottom of the hole. Never had a problem with rot when I did that.

Mike McCann, PE, SE (WA)

 

[medeek]

I was under the impression that PT wood was "good" with concrete, that is why we always use PT sill plates in conventional construction.

If you put 6" of gravel in the bottom of the hole I'm assuming that you use some type of collar (wood or concrete) otherwise the vertical bearing pressure of the posts would punch right down through that gravel. Everything I've seen so far usually calls out at least 6" of concrete in the bottom of the hole as a footing for the posts. The NFBA manual states that any friction between the posts and soil cannot be considered for vertical bearing.

With regards to the lateral load applied to the cantilevered posts: Do I mass all of the load from the roof and walls (trib. load by area for that post, transverse direction) into a point load at the eave height?

or

Do I apply a point load for the roof load at the eave height and then a rectangular distributed load from the walls to the full height of the post from the eave to grade?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

I'd go with option 2. It gives you the best chance of actually working

 

[Splitrings]

There are many different PT treatments and treatment levels. Some treatments have a long history of proven performance and others are questionable at best. Typically an oil borne treatment, such as Copper Naphthenate and Creosote, provide better long term performance. CCA is a water borne treatment that has a proven track record. Then there is the level of treatment that should be specified for the end use. The AWPA has treatment categories by end use. By specifying UC4C - Critical Structural Components, only the treatments that are suitable will be used. Most importantly, you won't find these treatments at Lowe's or Home Depot. It will have to be ordered.

 

[medeek]

I've decided to use the rigid diaphragm method as explained in the thesis by Drew Patrick Mill (August 2012). However with this method it is assumed that the endwalls provide the primary resistance to the load on the diaphragm (eave loading). If there is a significant opening one end of the structure then I'm assuming the posts on each side of that opening will act like segmented shearwall chords and should be embedded and sized such that they can resist the uplift and compression in this role. For example I have a 20ft end wall with a 12ft opening, 4ft of wall on each side of the door.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

For a shearwall with a higher unit shear typically I would call out a tighter nail spacing (ie. 4" or 3" on center spacing) for walls sheathed with wood. However, I'm looking at a couple of plans by other engineers and architects where the pole frame structure is metal on wood and I don't see any specific call outs for a higher fastener count or additional girts for the endwall with the large opening and therefore higher unit shears. Am I missing something here?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[Splitrings]

I guessing that most "pole structures" are not designed with diaphragms.

 

[jayrod12]

It's definitely what splitrings said.

Most shearwall calcs don't work as the posts are spaced too far apart for the plywood to act properly.

And the metal diaphragms don't have many published values and the ones I've seen lead me to assume they're only good for 100plf and don't screw around with the fastening (pun intended).

All of the ones I've seen designed are cantilevered columns.

 

[medeek]

This post frame engineering is not as simple as I thought it would. How the heck do these things get actually engineered?

I guess with this particular case I will have to assume that the endwall shear is simply taken up by the two endwall posts, cantilevered.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[Splitrings]

A first order analysis is done as a portal frame. Two posts and a truss in between, with or without knee braces. A second order analysis can be done by moment distribution, stiffness method, etc.

 

[medeek]

Has anyone solved or have in their knowledge base somewhere the correct expression for the maximum positive moment in a pin/roller propped cantilever. I have the max. negative moment per Drew Mill's paper and a nifty derivation he did using slope-deflection equations. Unfortunately he did not solve for the positive moment as well.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[Splitrings]

If you are looking at the embedded section of the pole, ASABE EP486 has the equations.

 

[medeek]

This is above ground. Basically this is the equivalent of a cont. beam with two unequal spans with a uniform load on one span.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Specifically I am looking for M₁ of Figure 26 of the AWC Design Aid #6. The only difference is that the two spans are not equal. I've searched everywhere and cannot find a derived solution to this problem. I guess I will have to work it using the stiffness method, time to open up my structural analysis book...

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

As ugly as the analysis is. The pole sheds that I have actually got to calc out I had to assume fixed at the base and a spring at the top.

I then played with the spring constant until the diaphragm shear was within my allowable.

It never ends up being worth the fees but that's how I had to do it.

 

[medeek]

Sporadically plugging away at a spreadsheet that hopefully can completely calculate a post frame building. This is what I have so far:

http://design.medeek.com/resources/postframe/2015-009_POST_FRAME_DESIGN.pdf

As you can see I'm still establishing the basic loads and deflections, however in a few hours I should be able to determine if this particular structure will actually "work". The kind of cool thing with this particular calculator is that I've integrated the wind load calculator into a separate sheet so changing wind loads is a "breeze".

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

The NFBA manual in section 8.3.5 suggests that one can take an additional twenty percent increase in allowable vertical soil pressure for each additional foot of depth, and up to a max. of three time the original value. However, I am not seeing this allowance for an increase anywhere in Section 1806 of the IBC, am I missing something?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[Splitrings]

In IBC 2003 it is in a footnote to table 1804.2. In IBC 2012 it is in section 1806.1. These increases are only allowed for load combinations that include wind or earthquake loads.

 

[medeek]

Everything seems to calc out okay until I get to the bending moments at the base of the central posts (see second to last page of pdf). The problem is I'm dealing with an "as built" so I can call out a 6x8 DF No. 1 post, I'm stuck with a 6x6 PT post that is probably a HF No. 2.

Current Spreadsheet and PDF output is here:

http://design.medeek.com/resources/postframe/

The ultimate wind speed is 155 mph (120 mph ASD) or 100 mph fastest mile. I've taken a reduction of .87 in the wind speed in order to bring this down to a Risk Category I building, which drops my ultimate wind speed to 135 mph.

Even so the combined stress on the 6x6 HF No. 2 central posts (windward side) are at 151% of the allowable.

What else more can I do to make this building work?

I've gone through the spreadsheet fairly carefully looking for any calculations errors or flaws in the methodology but it appears to be mostly sound.

Granted I haven't checked uplift yet and C&C out of plane loads to the roof and wall sheathing but that will probably be fine based on previous experience.

I've also taken a vertical pressure increase per ASAE486.1 for the footings since the jurisdiction has not adopted the more recent codes and their code books still reference this document and not the more recent ASAE486.2.


The really big problem I have if I can't get the numbers to work is that I probably will not get paid for this job. The client does not want to hear that his RV garage has to be torn down because my numbers don't work. Even if one was to retrofit the central posts by sistering 2x6's onto the appropriate face, this would still not really help with the max. moment occurring right at the ground line. If I can't get it to work the customer will probably hire another engineer who can make it work.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com